Hawaiian landslides have been catastrophic

Volcanic activity and gentle erosion have not been the only forces to shape the Hawaiian islands.
Landslide debris has now been mapped off of all the islands. Enormous amounts of material have
traveled great distances, indicating that the slides were truly catastrophic. The Nuuanu and Wailau
landslides, shown in the map, tore the volcanoes forming eastern Oahu and
northern Molokai, respectively, in half, and
deposited blocks large enough to have been given names as seamounts. Tsunamis generated
during these slides would have been devastating around the entire Pacific Basin. (See the coral reefs page for evidence that an enormous tsunami hit the shores of Lanai.)

Our research on Hawaiian landslides

Mauna Loa's unstable western flank

MAUNA LOA - Four new remotely operated vehicle dives carried out by Monterey Bay Aquarium Research Institute (MBARI) reveal a heterogeneous distribution of lithologies and compositions along a transect across the submarine west flank of Mauna Loa, from the outer scarp of the frontal bench to the upper flank. The frontal bench is composed predominantly of volcaniclastic sediments, ranging from very fine-grained monomictic hyaloclastites to coarse-grained, compositionally mixed volcaniclastic breccias. The predominance of subaerially derived clasts suggests accumulations of landslide deposits, probably emplaced along a regional shear plane preserved in cataclastic breccias with local foliations and grain trails. Repeated packages of inversely graded strata are interpreted to reflect thrust imbrication of the resulting volcaniclastic apron during volcanic spreading of Mauna Loa's western flank, similar to that now documented along Kīlauea's south flank. Many of the rocks from the bench show evidence for alteration, ranging from low-grade burial diagenesis to higher-grade hydrothermal alteration, including phases never before observed in submarine Hawaiian rocks, including epidote, talc, sphene, and corrensite. Alteration is concentrated in deformed zones, denoting pathways for fluid flow into or out of the volcanic edifice. Formed at depth, the altered rocks were subsequently transported along low-angle thrust faults into the bench and exposed along high-angle fractures and faults. The upper submarine flanks are draped by subaerially erupted, submarine emplaced pillow lavas and interbedded hyaloclastites, generated by shoreline-crossing lava flows. Basalt glasses indicate Mauna Loa origin but imply earlier compositions than present-day lavas, consistent with Ar-Ar ages suggesting eruption 0.28 ± 0.10 Ma. Late stage detachment of a nearshore slump produced the 'Ālika 2 debris avalanche that broke through the frontal bench, perhaps portending the evolution of the active Hilina slump now present on Kīlauea volcano's south flank.

Spreading of Mauna Loa's flank

MAUNA LOA - A transect of four ROV Tiburon dives
across the submarine west flank of Mauna
Loa volcano yields compelling evidence for volcanic spreading and associated
hydrothermal circulation during volcano growth. A frontal bench at the toe of
the flank, formerly thought to be a downdropped block of Mauna Loa, contains a
mix of volcaniclastic lithologies, including distally derived siltstone,
mudstone, and hyaloclastite. The bench is overlain by bedded gravels and
subaerially erupted pillow flows derived from local shoreline-crossing lava
flows. The volcaniclastic strata in the bench were offscraped, uplifted, and
accreted to the edge of the flank, as it plowed seaward into the surrounding
moat. The accreted strata underwent significant diagenesis, through deep burial
and circulation of hydrothermal fluids expelled from porous sediments beneath
the volcano. Timing constraints for bench growth and breakup suggest that
catastrophic failure of the subaerial edifice ca. 250–200 ka triggered volcanic
spreading by reducing stresses resisting basal sliding and rift-zone inflation.
Increased eruptive activity, and westward migration of Mauna Loa's southwest
rift zone, gradually rebuilt the massive flank, arresting slip prior to
detachment of the Alika 2 debris avalanche ca. 120 ka.

Geologic history of Wai'anae Volcano

OAHU - Wai'anae Volcano comprises the western half of O'ahu Island, but until recently
little was known about the submarine portion of this volcano. Seven
submersible dives conducted in 2001 and 2002, and multibeam bathymetry offshore
of Wai'anae provide evidence pertaining to the overall growth of the volcano's
edifice as well as the timing of collapses that formed the Wai'anae slump
complex.

A prominent slope break at ~1400 meters below sea level marks the paleoshoreline of Wai'anae at
the end of its shield-building stage and wraps around Ka'ena Ridge, suggesting
that this may have been an extension of Wai'anae's northwest rift zone.
Subaerially erupted tholeiitic lavas were collected from a small shield along
the crest of Ka'ena Ridge, now submerged. To the south, tholeiitic pillow lavas
have been recovered 65 km from the volcano's center, indicating the
south rift zone extended at least this distance. Sediment cores collected
from north of Ka'ena Ridge contain pelagic sediment with volcaniclastic grains
and volcanic glass that originated from Wai'anae's postshield stage and eastern
Oahu's Ko'olau Volcano's shield stage, respectively.

Multiple collapses and deformation events occurred during and after the
shield stage, resulting in compound mass wasting features on the volcano's
southwest flank, the Wai'anae slump complex. This slump complex is the largest in Hawai'i,
covering an area of ~5500 km2. It is composed of several
distinct sections based on morphology and lithologies of collected samples. The
outer bench of the slump complex contains tholeiites that correlate with
subaerial lavas erupted early during the volcano's shield stage, from 3.9 to 3.5
million years ago (Ma), and probably formed during and shortly after the early shield
stage. To the southwest of the outer bench lies a broad debris field of
subaerially derived volcaniclastic rocks containing tholeiites with early shield
compositions, interpreted to have formed by a
catastrophic collapse event that breached the outer bench. The breach may have
then been filled by slumping material from the main volcanic edifice. Finally, on top of
the northern main body of the slump is a rotated landslide block that detached
from the proximal part of the Ka'ena Ridge after the volcano's late shield stage
(3.2 to 3.0 Ma), containing higher alkali rocks that correlate with late shield-stage subaerial
lavas. None of the slump complex samples correlate with
alkalic subaerial postshield lavas.

Slope failure on Kilauea's submarine south flank

KILAUEA - Observations along the submarine south flank of Kilauea volcano have revealed the
subsurface structure of active submarine slope failure and the remnants of an ancient
landslide. New multichannel reflection data and high-resolution bathymetry provide
this evidence, and suggest a dynamic interplay among slope failure, regrowth, and
volcanic spreading. Disrupted strata along the upper reaches of Kilauea's flank
denote a coherent slump, correlated with the active Hilina fault zone on land.
The slump comprises mostly slope sediments, underlain by a detachment 3-5 km deep.
Extension and subsidence along the upper flank is compensated by uplift and folding
of the slump toe, which surfaces about midway down the submarine flank. Uplift of
strata forming Papa'u seamount and offset of surface features along the western
boundary of Kilauea indicate that the slump has been displaced ~3km in a south-southeast
direction. This trajectory matches coseismic and continuous ground displacements
for the Hilina slump block on land, and contrasts with the southeast vergence of
the rest of the creeping south flank. To the northeast, slope sediments are thinned
and disrupted within a recessed region of the central flank due to catastrophic
slope failure in the recent past. Debris from the collapsed flank was shed into
the moat in front of Kilauea, building an extensive apron. Seaward sliding of
Kilauea's flank offscraped these deposits to build an extensive frontal bench.
A broad basin formed behind the bench and above the embayed flank. Uplift and
back tilting of young basin fill indicate recent, and possibly ongoing, bench
growth. The Hilina slump now impinges upon the frontal bench; this buttress
may tend to reduce the likelihood of future catastrophic detachment.

Conditions for landslides and canyon formation

MOLOKAI - The main break-in-slope on the northern submarine flank of Molokai at
1500 to 1250m depth is a shoreline feature that has been slightly modified by
the Wailau landslide. Submarine canyons above the break-in-slope were
subaerially carved. Where such canyons cross the break-in-slope, plunge
pools may form by erosion from bedload carried down the canyons.

West Molokai Volcano's continued infrequent eruptions formed a series of small
coastal sea cliffs, now submerged, as the island subsided. Lavas exposed
at the break-in-slope are subaerially erupted and emplaced tholeiitic
shield lavas. Submarine rejuvenated-stage volcanic cones formed after the
landslide took place and following at least 400-500m of subsidence after
the main break-in-slope had formed. The sea cliff on east Molokai is not
the headwall of the landslide, nor did it form entirely by erosion. It may
mark the location of a listric fault similar to the Hilina faults on
present-day Kilauea Volcano. The Wailau landslide occurred about 1.5 Ma
and the Kalaupapa Peninsula most likely formed 330 +5ka. At their
peak, West and East Molokai stood 1.6 and 3 km above sea level.

High rainfall causes high surface runoff and formation of canyons, and
increases groundwater pressure that during dike intrusions may lead to
flank failure. Active shield or postshield volcanism (with dikes injected
along rift zones) and high rainfall appear to be two components needed to
trigger the deep-seated giant Hawaiian landslides.

Improvements in mapping landslides

OAHU, MOLOKAI - The development of ideas on the giant Hawaiian landslides parallels
improvements in the technology of bathymetric mapping and navigation. The
landslides were first recognized in the 1960s in a relatively detailed
U.S. Navy single-beam sonar survey utilizing an improved radio navigation
system. The GLORIA multibeam side-scan sonar system (1980s) imaged
unprecedented detail in the known landslides and revealed numerous other
undiscovered ones. The JAMSTEC multibeam surveys (late 1990s), utilizing
GPS navigation, produced detailed maps of the entire landslide area for
the first time.

Volcaniclastic rocks on the flanks of landslide blocks

OAHU, MOLOKAI - The rocks exposed on the steep slopes of giant landslide blocks in the
Nuuanu and Wailau landslides are fragmental rocks: hyaloclastite and
volcaniclastic breccias. They form as 1) secondary slope mantling of
unlithified breccia consisting of clasts in a mud matrix; 2) hyaloclastite
and breccia, all with zeolite cement, that form downslope of the shoreline
where lava flows enter the sea and fragment; and 3) breccia formed by
tectonic fragmentation of glassy submarine-erupted pillow basalt. Lavas
erupted from single volcanoes are highly variable in mayor-element
composition, even during their tholeiitic shield stage, making it
difficult to identify which landslide block was derived from which
volcano. Low-temperature fluids circulate through the fragmental deposits
on the flanks of the volcanoes, partially altering the glass to palagonite
and cementing the volcaniclastic rocks with Na- and K-rich zeolites.
Spreading of the volcano early in its history along low-angle thrust
faults laterally transports deep submarine pillow lava into the flank of
the volcano where it crops out as tectonic breccia. The faults underlying
the landslide blocks are within this tectonized core of the volcano, not
simply within the shallow slope deposits of hyaloclastite and breccia. The
Nuuanu landslide predates the 1.5 Ma Wailau landslide.